FLUID INCLUSIONS WITH GAS BUBBLES AS GEOTHERMOMETERS GEOL RUNDSCH 42:19-34

Abstract

Two types of fluid inclusions can be distinguished. The first is based on the assumption of Sorby (1858) that a homogeneous phase, such as water, salt solution, or CO2, is entrapped, meaning that the bubbles result from the gas of the enclosed fluid. The second type includes foreign gas entrapped with the fluid. "Sorby"- type inclusions can be used as thermometers if either the formation pressure is known, or the pressure impact has been shown to be unimportant. It is only possible to neglect pressure in cases where the degree of filling is high enough that bubbles vanish at low temperatures. If, during entrapment, the fluid included dissolved foreign gas that was released during cooling, it is even more dangerous to equate filling with formation temperatures. Compared with Sorby-type inclusions, even less information is available about the expected large-pressure impact. If foreign gas was entrapped as bubbles, the filling temperature may significantly deviate from the formation temperature and may even increase at first during heating. Such non-Sorby-type inclusions can be identified by measuring the degree of filling and comparing the related filling temperature with that of water and CO2, respectively, at a similar degree of filling. The question of the composition of the inclusions is important not only for determining temperatures, but also for solving questions about the formation and alteration of rocks. Attention is drawn to the method of Brewster (1826), who determined the refractive index by using the total reflection. All these remarks are valid for both primary and secondary inclusions. The decrepitation method is not an appropriate means to distinguish primary from secondary inclusions. Further, it is not suited to determine the type of inclusion, or the degree of filling. Fluid inclusions with gas bubbles have been used as geothermometers for several years. Thus, it is necessary to test this method in some detail. Different opinions about this "geothermometer" have resulted from evaluations on fluid inclusions carried out several times during the last 100 years. In the present paper, current and new studies will be evaluated and a few new observations and instructions are presented. The discovery of fluid inclusions in crystals is relatively old. The nine epigrams of Claudian, a contemporary of the holy Augustin, gave the earliest observations of fluid inclusions. Claudian summarized as follows: De crystallo, cui aqua inerat [On the rock crystal that includes water](1824). Modern scientific studies of fluid inclusions began during the early 19th century. The famous English chemist, Sir Humphrey Davy, investigated the chemical composition of such inclusions in 1822. Sir David Brewster is known from the history of crystal optics. From 1823 onwards, he described inclusions with bubbles from several minerals including those with two different mobile fluids (Brewster 1823; Fig. 1). In 1858, H.C. Sorby explained the use of fluid inclusions as thermometers and provided a highly sophisticated and critical evaluation that is still worth reading. Sorby (1858) started from the principle that a homogeneous fluid, e.g. water or a diluted salt solution, had been entrapped in the crystal. As the system cools, the water will shrink yielding a vapor bubble. During reheating of the inclusion, the bubble will disappear at the entrapment temperature. Taking into account the relation between the volume of the gas bubble and the bulk volume of the inclusion, as well as the expansion coefficient of water, Sorby (1858) calculated the temperature at which the vapor bubble should vanish. To conveniently determine the exact entrapment temperature, particularly of fluid inclusions with irregular shapes, a heating stage should be used that is fixed at the microscope.